The present invention is directed to an apparatus for locating or detecting the absolute position of a body of interest without contacting the body by using a magnetic sensor. More particularly, the present invention is directed to a magnetic position detector having a long measuring range.
Recently, various types of non-contact distance measuring instruments have been developed for measuring linear and angular displacement and detecting absolute position of a body in linear or angular systems. There are various types of non-contact measuring position detecting apparatus. An instrument utilizing a sliding resistor potentiometer is a classic example, but is not reliable. An optical position device has an optical sensor which reads out an optical scale such as a slit scale, but the construction of the device is rather complicated. Further, there is a magnetic scale wherein a scale written into a magnetic medium is read out by a magnetic sensor. Its structure, however, is also complicated and an absolute position of a body cannot be measured, only the distance between two arbitrary points can be measured. The present invention is intended to provide a magnetic position detecting apparatus capable of detecting the absolute position of a body to be detected, having a fairly simple structure, a long measuring region, and high reliability.
FIG. 1 is a schematic perspective view of a prior art apparatus for detecting a position (hereinafter referred to as a position detector), wherein a permanent magnet 1 is arranged in parallel facing a magnetic sensor 2. The distance L between the magnet 1 and the magnetic sensor 2 is variable. That is, relative movement between both members is allowed. It should be noted that the magnetic sensor 2 is transferred in the direction of the pole axis of the magnet 1. The magnetic field emanating from the permanent magnet 1 is sensed by a sensitive magnetic sensing element in the magnetic sensor 2. The distance L is detected and output in the form of a signal from the magnetic sensor 2.
FIG. 2 is a graph of the relationship between the distance L and the output signal voltage of the magnetic sensor 2 of a prior art position detector. The coercive force of the permanent magnet 1 is 800 Oe and the magnetic sensing element of sensor 2 is made of a magnetoresistive thin film. The effective distance L.sub.eff (the portion of the curve which is fairly linear and available for practical measuring), is rather short, approximately 4 to 30 mm.
There are various types of elements employed in a magnetic sensor such as a coil for sensing an induced magnetic field, a hole-effect semiconductor element, and a ferro-magnetic magnetoresistive element. Among them, the magnetoresistive sensing element has the ability to change its resistivity in the presence of an external magnetic field. The element is stable with respect to changes in the environmental temperature and is suitable for sensing a micro-magnetic field. Before proceeding further, the structure of a magnetic sensor of this type, which is disclosed in Japanese Provisional Publication of the Patent Application SHO No. 61-70478 to Hirano, published Apr. 11, 1986, will be described briefly.
FIG. 3 is a partial plan view of a magnetic sensing element 5, and FIG. 4 is a cross-sectional view of the magnetic sensor 2. In FIG. 4, the magnetic sensing element 5 shown in FIG. 3 is patterned in a meandering pattern and formed from a magnetoresistive thin film of Permalloy (Fe-Ni) deposited on a silicon substrate 3 by lithographic technology. Thereafter, the patterned magnetoresistive element is subject to a heat treatment so as to be magnetized in a longitudinal direction. Then striped gold conductive layers 7 obliquely crossing the element pattern are deposited thereon, adhering to the magnetoresistive pattern through intermediate layers 6 of Ti or Cr, which are previously formed. The element formed in this configuration is referred to as a "barber-pole type" element, which is described in various references such as Electronic Components and Applications, page 148, Vol. 5, No. 3, June 1983, by U. Dibbern and A. Petersen. The sensor is then covered with a passivation film 8 for protection against environmental hazards and lead terminals 9 are formed on the substrate 3. The feature of the magnetic sensor proposed by Hirano is that the striped pattern is formed over all the paths of the meandering magnetic sensing element including both going and returning paths, while other conventional elements have the striped pattern formed on only one side of the path.
FIG. 5 is a schematic plan view of four magnetoresistive sensing elements of a barber-pole type, connected to form four arms of a Wheatstone bridge having a pair of input and output terminals and formed on substrate 3. The degree of bridge imbalance is used to indicate the magnetic field intensity which is indicated by the arrow Hex in the figure.
FIG. 6 is a graph of the characteristics of the barber-pole type magnetic sensor of FIGS. 3, 4 and 5, wherein the external magnetic field intensity Oe is plotted with respect to the abscissa and the output voltage mV of the sensor is plotted with respect to the ordinate. As can be seen in FIG. 6, the characteristic curve of the magnetic sensor is satisfactorily straight, and extends from the negative to the positive direction. This means that the barber-pole type magnetoresistive sensor has a sensing polarity peculiar to its magnetizing direction.
Since barber-pole type elements sense the magnetic flux only in parallel with the plane of the Permalloy strips, their sensitivity shows a sharp peak value when the magnetic sensing element 5 is positioned on a plane in parallel with the magnetic flux lines. This results in a fairly high resolution capability of the magnetic sensor for detecting the position of a body of interest. On the other hand, accurate transferring motion of the magnetic sensor is required so that the plane of the magnetoresistive sensing element is always kept in parallel with the plane of the magnetic flux lines to be sensed.
In the following description of the present invention, the magnetic sensor employed is assumed to have a barber-pole type magnetoresistive sensing element. Therefore, a common reference numeral (15) is used throughout for denoting the magnetic sensor appearing in the following description unless otherwise mentioned.
The prior art position detector, as described above, has a short sensing region. This is because the magnetic field to be sensed by the magnetic sensor 2 emanates in a direction perpendicular to the pole face of the permanent magnet 1. In other words, the magnetic sensor 2 moves in the direction of the magnetic pole axis. The magnetic field intensity in this direction, therefore, decreases very sharply with an increase in the distance from the pole surface, and is inversely proportional to the square of the distance L. Thus, a magnetic position detector having a longer sensing region is needed.